The present invention relates to a method of treating gases from a pressurized fluidized bed reaction system. The invention reduces nitrogen emissions in connection with pressurized fluidized bed combustion performed in a fluidized bed of solids at a pressure above atmosphere.
For many years the emission requirements of industrial power plants have been under exhaustive investigations. New energy producing methods have been established and commercialized, with ever increasing pollutants capturing facilities and efficiencies, in a cost-effective way. In particular, it has long been desired to find cost effective ways to minimize nitrogen based pollutants, nitrogen oxides, NO.sub.x, and nitric oxides, N.sub.2 O.
Nitrogen oxides can be formed during the combustion process mainly via three different reaction routes:
The first route is a direct oxidation of the molecular nitrogen by free oxygen radicals forming "thermic NO.sub.x ". The reaction route is, according to present knowledge, assumed to be as follows: EQU N.sub.2 +O=NO+N (1a) EQU N+O.sub.2 =NO+O (1b)
The formation of "thermic NO.sub.x " depends on the concentration of the free oxygen atoms in a combustion reaction. Free oxygen atoms are formed only at high temperatures, and it has been assumed that at temperatures below 1700 K the amount of "thermic NO.sub.x " is negligible in total NO.sub.x emission.
The second route is a reaction in a fuel rich zone between hydrocarbon radicals and molecular nitrogen forming HCN which is oxidated in the combustion chamber forming "prompt NO.sub.x ": ##STR1##
Reaction rates of reactions (2a) and (2b) do not depend strongly on the temperature, and it is assumed that only in cold, fuel rich conditions are significant amounts of NO.sub.x is formed via these reactions.
According to the third route, fuels contain nitrogen which is bound in fuel material and is released during the combustion process, forming NO, N.sub.2 O and N.sub.2. Part of this nitrogen is released in the form of HCN or NH.sub.3 with volatile matter, and part of nitrogen remains in char.
The homogenous reactions of HCN is considered to be the main source of nitrous oxide (N.sub.2 O) formed during combustion. The reaction route is: ##STR2##
Because NO.sub.x is mainly formed via oxidation of nitrogen compounds or nitrogen itself, the concentration of oxygen in the reactor has a clear effect on NO.sub.x emission in combustion. On the other hand, in low oxygen concentrations some carbon monoxide and other reducing agents may be formed which are known to reduce NO.sub.x and forming N.sub.2.
In Swedish patent application 18903891 it has been suggested to inject ammonia (NH.sub.3)into a pressurized fluidized bed reactor enclosed by a pressure vessel. The Swedish document suggests using an ammonia injection into flue gas in a pressure vessel before the gas turbine and after that catalytic reduction with additional injection of ammonia into the flue gases after the gas turbine. This document also teaches injection of additional ammonia based on measurement of the NO.sub.x -content after the gas turbine and before catalytic reduction. However this and other known methods of removing nitrogen based pollutants in Pressurized Fluidized Bed Combustion systems still have shortcomings.
According to the invention, it has been found that significant amounts of NO.sub.x can be reduced to N.sub.2 when NH.sub.3 (or a like reducing agent) is injected into hot flue gases at superatmospheric pressure (typically over 2 bar, preferably about 2 to 100 bar). When NH.sub.3 is injected at high enough temperatures, and the residence time for NH.sub.3 in hot conditions is long enough, undesired side effects--e.g. increase in N.sub.2 O, CO and NH.sub.3 emissions--can be almost totally avoided. This is especially so if the reducing agent is efficiently mixed with the gas and after that arranged to move slowly, e.g. at a velocity of about 1-20 cm/s (preferably about 1-5 cm/A) when passing through a particle separator.
According to one aspect of the present invention a method of purifying hot exhaust gases from a pressurized fluidized bed reactor system including a fluidized bed reactor within a pressure vessel, a separator for separating particulates from the exhaust gases, and a gas expansion device (e.g. turbine) for expanding the gas after separation of particles therefrom is provided. The method comprises the following steps: (a) Compressing gas to superatmospheric pressure. (b) Supplying the superatmospheric pressure gas to the fluidized bed reactor and pressure vessel so that the pressure within the pressure vessel is also superatmospheric. (c) Effecting chemical reactions in the fluidized bed reactor at superatmospheric pressure to produce hot exhaust gases containing gaseous impurities and particulates. (d) While maintaining superatmospheric pressure conditions, conveying the exhaust gases to the separator, effecting separation of particles from the exhaust gases with the separator to produce clean gas, and conveying the clean gas to the gas expansion device. And, (e) during the practice of step (d), introducing a reducing agent into the exhaust gases effective to reduce at least a significant proportion of the gaseous impurities in the-exhaust gases.
The gaseous impurities in the exhaust gases include nitrogen oxides, and step (e) is typically practiced to introduce a nitrogen oxides reducing agent, preferably NH.sub.3, or nitrogen containing compound, CO, CH.sub.4, or nitrogen producing compound. The particle separator typically includes a filtering surface on which a filter cake forms, and step (e) may be practiced between the fluidized bed and the filtrate cake, and also between the filtrate cake and the gas expansion device, or only between the flitrate cake and the gas expansion device. Step (e) may be practiced at a plurality of locations between the filtrate cake and the gas expansion device--for example where the separator comprises a plurality of clusters of filter elements, reducing agent may be injected at a location associated with each of the clusters.
Typically the pressure vessel comprises a first pressure vessel, and the separation device is mounted within a second pressure vessel exteriorly of and distinct from the first pressure vessel (the second pressure vessel also at superatmospheric pressure, preferably over 2 bar). Step (d) is also practiced to reduce the velocity of the exhaust gases between the first pressure vessel and the separation device so that the velocity of the exhaust gases when flowing the filtration device is about 1/10 to 1/1000 the velocity of the exhaust gases when leaving the fluidized bed. Typically the velocity is reduced so that when the exhaust gases flow through the filtration device their velocity is about 1-20 cm/s (preferably about 1-5 cm/s).
Under some circumstances it is desirable to introduce the reducing agent as or just before the clean gas exits the second pressure vessel, the velocity of the gas as it exits the second pressure vessel increasing significantly (at least doubling, and typically increasing to a value of about 10-1,000 times the velocity when passing through the separation device), so as to provide efficient mixing between the clean gas and reducing agent immediately after introducing of the reducing agent.
Step (e) is also preferably practiced so that the amount of introduced reducing agent is substantially only the minimum amount necessary to effect reduction of the gaseous impurities, so that there is no significant waste of reducing agent. Because of the pressurized conditions, small gas velocity, and particular points of introduction of reducing agent, provided according to the present invention, this desired result can be readily achieved.
According to another aspect of the present invention a method of purifying hot exhaust gases, having NO.sub.x and particles therein, from a PCFB (pressurized circulating fluidized bed) combustor is provided. The method utilizes a separator for separating particles from the exhaust gases contained within a pressure vessel, the separator having a plurality of filter surfaces each having a clean side and a dirty side. The method comprises the steps of: (a) Introducing flue gas from the PCFB combustor to the dirty sides of the filter surfaces in the pressure vessel. (b) Separating solid particles from the gas so that a flitrate cake builds up on the dirty sides of the filter surfaces. (c) Introducing NO.sub.x reducing agent into the gas associated with the clean sides of the filter surfaces. (d) Providing an optimized retention time of NO.sub.x reducing agent in the gas so as to optimize NO.sub.x reduction. And, (e) exhausting the gas, after the practice of steps (c) and (d), from the pressure vessel.
As indicated above the pressure in the pressure vessel is typically over 2 bar, preferably about 2 to 100 bar. That is step (d) is practiced by maintaining the pressure vessel at superatmospheric pressure of at least 2 bar. Step (d) is also further practiced by reducing the velocity of the gas substantially immediately after introduction into the pressure so that it is about 1/10 to 1/1000 the velocity of the gas prior to introduction into the pressure vessel; that is step (d) is further practiced to cause the gas to flow at a flow rate of about 1-20 cm/s (preferably about 1-5 cm/s) as it passes through the filter surface and prior to step (e).
According to another aspect of the present invention, a system for removing gaseous impurities and particles from hot gases is provided comprising the following elements: A pressure vessel at superatmospheric pressure and having a gas inlet and a gas outlet. A PCFB connected to the gas inlet. A plurality of filter elements mounted within the pressure vessel between the inlet and outlet, each filter element having a filter surface having a dirty side on which filtrate cake forms, and a clean side, the dirty side in communication with the gas inlet, and the clean side in communication with the gas outlet. And, at least one injector for injecting reducing agent into the pressure vessel between the clean sides of the filter surfaces and the gas outlet.
The system further comprises means for reducing the velocity of the gas introduced into the gas inlet so that the gas has a velocity of about 1-20 cm/s (preferably 1-5 cm/s) when flowing through the filter surfaces. The gas velocity reducing means may comprise an introduction duct provided within the pressure vessel between the gas inlet and the filter elements, for example providing a much larger volume than the conduit that the gas flows in prior to passage into the gas inlet so that the gas velocity is dramatically reduced. A turbine or a like gas expansion means is also connected to the gas outlet.
The at least one injector may comprise an injector associated with each of the filter elements; and/or an injector for injecting reducing agent into the gas at or just prior to where the gas exits the pressure vessel through the gas outlet, the gas outlet being constructed so that the velocity of the gas exits the gas outlet at least doubles so as to provide good mixing of reducing agent with the gas. The filter elements may comprise any suitable filter elements that can withstand the high temperature of the gases (which are typically always over 300.degree. C., and may be as high as 1200.degree. C.); suitable presently existing filter elements that can be used include ceramic candle filter elements and ceramic honeycomb filter elements, both of which are conventional per se.
The combination of the filtrate cake forming on the filtering surface, the superatmospheric pressure, and the relatively small velocity of the gas passing through the filtration cake, increase the retention time of the gaseous impurities associated with and in contact with the reducing agent, giving more time for purification chemical reaction as well as efficient mixing of the agent with the gaseous impurities.
It is the primary object of the present invention to provide an effective manner of purifying hot exhaust gases from a pressurized fluidized bed reactor system, particularly the removal of NO.sub.x therefrom, in an efficient manner, without substantial increase in N.sub.2 O, CO, or NH.sub.3 emissions. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.